This progam uses VCL, the default UI components library of Delphi, to print "Hello World" into a message box. The VCL wrapps most of the commonly used WinAPI components. This way, they can be used much easier, e.g. without the need to work with Window Handles.
To include a dependency (like
Vcl.Dialogs in this case), add the
uses block including a comma-separated list of units ending with an semicolon.
program HelloWindows; uses Vcl.Dialogs; begin ShowMessage('Hello Windows'); end.